JP3519870B2 - Liquid crystal display - Google Patents

Description

Description: BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to a liquid crystal device which converts a DC power supply voltage supplied from the outside with a DC-DC converter and supplies a driving voltage to a liquid crystal driving circuit. It relates to a display device. 2. Description of the Related Art A liquid crystal display device connected to an external device such as a personal computer through a connector includes a device that receives a supply of a liquid crystal driving voltage from the external device and an internal device that uses a DC-DC converter or the like. Some are generated by In the case of a liquid crystal display device that receives a liquid crystal drive voltage from an external device as in the former case, the external device is provided with a liquid crystal drive voltage generation circuit, so even if a power failure or failure of the external device occurs, This can be dealt with quickly internally, but cannot be done quickly in the case of a liquid crystal display device that internally generates a liquid crystal driving voltage as in the latter case. In a conventional liquid crystal display device in which a voltage of a DC power supply VDD supplied from an external device is converted by a DC-DC converter and a driving voltage is supplied to a liquid crystal driving circuit, it is determined that the DC power supply VDD has risen to a predetermined voltage. DC-DC on the condition that a predetermined signal among the display control signals, for example, a frame signal, has arrived.
The operation of the converter is started, and the operation of the DC-DC converter is stopped when it is detected that the DC power supply VDD has dropped to a predetermined voltage (Japanese Patent Application No. Hei 7-22438).
No. 9). [0003] As described above, conventionally,
Since the operation of the DC-DC converter is stopped only by detecting that the DC power supply VDD has dropped to a predetermined voltage,
The voltage for driving the liquid crystal supplied from the DC-DC converter falls later than the fall of the DC power supply VDD, which causes a problem that electric charges remain in the liquid crystal panel and the panel deteriorates due to the residual charges. . Therefore,
SUMMARY OF THE INVENTION It is a main object of the present invention to prevent a liquid crystal driving voltage generated by a DC-DC converter incorporated in a liquid crystal display device from remaining on the liquid crystal driving voltage to prevent the panel from deteriorating. According to the present invention, there is provided a liquid crystal panel, a driving circuit for driving the liquid crystal panel based on a display data signal and a display control signal supplied from outside, and an external supply. In a liquid crystal display device including a power supply circuit for converting a voltage of a DC power supply by a DC-DC converter and supplying a drive voltage to the drive circuit, the power supply circuit controls operation of the DC-DC converter. A voltage detection circuit for detecting whether or not the input DC power supply voltage is equal to or higher than a predetermined value, connecting the output of the voltage detection circuit to a data terminal and a clear terminal, and transmitting a frame signal to the operation control circuit. An operation control circuit comprising: a flip-flop connected to a clock terminal; and a control signal monitor circuit for detecting the presence or absence of a signal whose output changes periodically. Controls the start and stop of the operation of the DC-DC converter based on the output of the flip-flop and the control signal monitor circuit. An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram of a liquid crystal display device according to an embodiment of the present invention. The liquid crystal display device includes a liquid crystal panel 1, a driving circuit 2 for scanning and signals of the liquid crystal panel connected thereto, a personal computer (not shown), and the like. A plurality of bias voltages (VH, V0, VM, V1, VL) and a logic voltage (VLCD) are supplied to the drive circuit 2 as drive voltages based on a DC power supply VDD supplied from a data processing external device via a cable. , GND), and a receiving circuit 4 which receives a display data signal and a display control signal provided from an external device via a cable and selectively supplies the display data signal and the display control signal to the drive circuit 2 and the power supply circuit 3. A connector 5 for connection to an external device is provided. The liquid crystal panel 1 can use a pair of substrates having electrode groups orthogonal to each other with a liquid crystal layer interposed therebetween. For example, a field effect type liquid crystal such as a super twisted nematic liquid crystal display can be used. The electrodes of the liquid crystal panel 1 constitute a so-called simple matrix and have no active elements at pixel intersections, but may have active elements. The drive circuit 2 can be configured by an integrated circuit element operable at a logic power supply voltage of +3 to +5 volts. As shown in FIG. 2, the power supply circuit 3 is supplied with a DC power supply [VDD (5 volts) -GND (0 volts)] from an external device, and supplies it to a plurality of DC voltages (VH, VA).
H, VAL, VL, VLCD, GND)
C-DC converter 6 and this DC-DC converter 6
An operation control circuit 7 for controlling on / off of the DC
A plurality of bias voltages (VH, V0, VM, V1, V1, V2, V3) supplied to the drive circuit 2 after receiving the output voltage of the DC converter 6;
VL) is provided. An operation control circuit 7 for controlling the start and stop of the operation of the DC-DC converter 6 includes, as shown in FIG.
The input DC power supply voltage VDD is a predetermined value (eg, 2.3 volts)
A voltage detection circuit 71 for detecting whether or not the above is the case, and an output S1 of the voltage detection circuit 71 is connected to a data terminal D and a clear terminal CL.
R and one of the display control signals
A flip-flop 72 connected to the clock signal CLK and a detection means for detecting and outputting a signal whose output changes with a periodicity, for example, the clock signal CL1 among the display control signals. A signal monitor circuit 73 is provided, and an AND circuit 74 is provided which outputs the output S2 of the monitor circuit 73 and the output S1 of the voltage detection circuit 71 under AND conditions. The operation of the above configuration will be described with reference to FIG. With the start of the operation of the external device, supply of DC power from the external device is started at time t1, and when the voltage VDD rises to a predetermined value at time t2, the voltage detection circuit 71 shown in FIG. The output S1 is switched from L level to H level. When the output S1 of the voltage detection circuit 71 goes high, the flip-flop 72 is activated. Thereafter, display data signals (DU0-11, DL0-11) and display control signals (frame signal FLM, clock signals CL1, CL2,
The polarity inversion signal M, the contrast signal VCON, etc.) are received by the receiving circuit 4. A part of the received display control signal is also sent to the power supply circuit 3, and at time t3, the control signal monitoring circuit 73 in the DC-DC operation control circuit 7 detects the presence of CL1 at the arrival of the control signal. As a result, the output S2 switches from the L level to the H level. At about the same time, the clock terminal CL of the flip-flop 72
When the frame signal FLM is input to K, the output Q of the flip-flop 72 switches from L level to H level. When both the output Q of the flip-flop 72 and the output S2 of the control signal monitor circuit 73 switch to H level, the output of the AND circuit 74 switches from L level to H level. O of DC-DC converter 6
When the H level output of the AND circuit 74 is given to the N / OFF terminal, the DC-DC converter 7 starts operating. When the DC-DC converter 6 starts operating, its output voltage rises by time t4, and a part of the output voltage is supplied to the drive circuit 2 via the bias circuit 8. The drive circuit 2 drives the display of the liquid crystal panel 1 based on the display data signal and the control signal sent from the reception circuit 4. When the transmission of the display data signal and the control signal is stopped at time t5 due to a power failure or a failure of an external device, the control signal monitor circuit 73 detects this and outputs the signal at a time slightly delayed from time t5. At t6 from H level to L
Switch to level. When the output S2 of the control signal monitor circuit 73 switches to the L level, the output of the AND circuit 74 switches from the H level to the L level, which is given to the ON / OFF terminal of the DC-DC converter 6, The operation of the DC-DC converter 6 stops. With the operation stop of the DC-DC converter 6,
Various voltages supplied from the power supply circuit 2 to the liquid crystal panel 1 drop and reach the GND level at time t7. Therefore, the voltage detection circuit 71 detects that the input power supply voltage VDD has dropped to a predetermined voltage, and the DC-D
Compared to the case where the operation of the C converter is stopped, the voltage VDD
The operation of the DC-DC converter 6 can be started earlier by an amount of time corresponding to the time required until the voltage of the liquid crystal panel 1 decreases to a predetermined voltage. Can be prevented. In particular, when the logic power supply voltage VLCD is set to a low voltage of about 3 volts and the input power supply voltage VDD is supplied at a low voltage of about 3 volts, the time during which this low voltage drops to the GND level is shortened, and the voltage is reduced. DC-DC after detecting drop
Since there is no time margin required for stopping the operation of the converter, it is advantageous to detect the reception stop of the display control signal and stop the operation of the DC-DC converter in that the time margin can be secured. In order to reliably detect the stop of the display control signal, the control signal monitoring circuit 73 controls the clock signals CL1, CL whose output changes periodically.
2. It is preferable to detect the presence or absence of the frame signal FLM, the polarity inversion signal M, and the like. The detection of the presence or absence of the high-frequency clock signal CL1 or the clock signal CL2 as in the embodiment quickly detects the stop of those signals. It is preferable because it can be performed. As described above, according to the present invention, when a liquid crystal driving voltage is generated by a DC-DC converter incorporated in a liquid crystal display device, the liquid crystal driving is performed when a power failure or a failure of an external device occurs. It is possible to prevent the panel from deteriorating due to residual use voltage.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a liquid crystal display device according to an embodiment of the present invention. FIG. 2 is a circuit diagram of a power supply circuit according to an embodiment of the present invention. FIG. 3 is a circuit diagram of a control signal monitor circuit according to the embodiment of the present invention. FIG. 4 is a time chart of a voltage according to the embodiment of the present invention. [Description of Signs] 1 Liquid crystal panel 2 Drive circuit 3 Power supply circuit 4 Receiving circuit

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-1-177076 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G02F 1/133 520 G09G 3/36

Claims (1)

(1) A liquid crystal panel, a driving circuit for driving the liquid crystal panel based on a display data signal and a display control signal supplied from the outside, and a DC power supply supplied from the outside In a liquid crystal display device provided with a power supply circuit for converting a voltage of the DC-DC converter by a DC-DC converter and supplying a drive voltage to the drive circuit, the power supply circuit includes an operation control circuit for controlling the operation of the DC-DC converter. Having the operation control
The circuit detects whether the input DC power supply voltage is above a specified value
Voltage detection circuit, and an output of the voltage detection circuit
Connector and clear terminal, and
Flip-flop connected to the clock terminal and the output is periodic
Control signal monitor that detects the presence or absence of a signal that changes with
Circuit, and the operation control circuit includes the flip-flop circuit.
And the output of the control signal monitor circuit.
A liquid crystal display device for controlling start and stop of operation of a DC-DC converter .